The Ocellated Ice Fish lives in the freezing waters of the Antarctic Ocean, where it manages to keep its body doing all the things that other fish do, but with blood that is absolutely clear, researchers told AFP on Friday.

The reason, say experts at Tokyo Sea Life Park, is that the Ocellated Ice Fish has no haemoglobin, making it unique among vertebrates the world over.

Haemoglobin is the protein found in every other animal with bones. It is what makes blood red and is the agent that carries oxygen around the body.

The fish, which has no scales, is a prize catch for the aquarium, the only place on the planet that has the curious specimen in captivity.

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Researchers believe the fish can live without haemoglobin because it has a large heart and uses blood plasma to circulate oxygen throughout its body.

Its skin is also thought to be able to absorb oxygen from the rich waters of the Antarctic, where it is found at depths of up to a kilometre (3,300 feet).